gels-07-00030-v2.pdf

gels
Review
Cellulose-Based Hydrogels for Wastewater Treatment:
A Concise Review
Maimuna Akter 1,†, Maitry Bhattacharjee 2,†, Avik Kumar Dhar 2, Fahim Bin Abdur Rahman 1,3 ,
Siddika Haque 4 , Taslim Ur Rashid 5 and S M Fijul Kabir 5,*

Citation: Akter, M.; Bhattacharjee,
M.; Dhar, A.K.; Rahman, F.B.A.;
Haque, S.; Rashid, T.U.; Kabir, SMF.
Cellulose-Based Hydrogels for
Wastewater Treatment: A Concise
Review. Gels 2021, 7, 30. https://
doi.org/10.3390/gels7010030
Received: 4 January 2021
Accepted: 16 March 2021
Published: 18 March 2021
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Copyright: © 2021 by the authors.
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This article is an open access article
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Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1 Department of Environmental Management, Independent University Bangladesh, Dhaka 1229, Bangladesh;
maimuna_041@yahoo.com (M.A.); frahman@email.sc.edu (F.B.A.R.)
2 Department of Textile Engineering, Shyamoli Textile Engineering College, University of Dhaka,
Dhaka 1207, Bangladesh; maitry1992moni@gmail.com (M.B.); avikkumardhar1990@gmail.com (A.K.D.)
3 Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA
4 Faculty of Textile Engineering, BGMEA University of Fashion and Technology, Dhaka 1230, Bangladesh;
siddika@buft.edu.bd
5 Wislon College of Textiles, North Carolina State University, Raleigh, NC 27606, USA; turashid@ncsu.edu
* Correspondence: skabir@ncsu.edu
† These authors contributed equally to this work.
Abstract: Finding affordable and environment-friendly options to decontaminate wastewater gen-
erated with heavy metals and dyes to prevent the depletion of accessible freshwater resources is
one of the indispensable challenges of the 21st century. Adsorption is yet to be the most effective
and low-cost wastewater treatment method used for the removal of pollutants from wastewater,
while naturally derived adsorbent materials have garnered tremendous attention. One promising
example of such adsorbents is hydrogels (HGs), which constitute a three-dimensional polymeric
network of hydrophilic groups that is highly capable of adsorbing a large quantity of metal ions
and dyes from wastewater. Although HGs can also be prepared from synthetic polymers, natural
polymers have improved environmental benignity. Recently, cellulose-based hydrogels (CBHs) have
been extensively studied owing to their high abundance, biodegradability, non-toxicity, and excellent
adsorption capacity. This review emphasizes different CBH adsorbents in the context of dyes and
heavy metals removal from wastewater following diverse synthesis techniques and adsorption mech-
anisms. This study also summarizes various process parameters necessary to optimize adsorption
capacity followed by future research directions.
Keywords: wastewater; heavy metal; dye; hydrogel; cellulose
1. Introduction
Water is the most abundant natural resource on the planet; however, only about 3%
of current water reserves are freshwater, while less than one-third of this freshwater is
usable for different household, agricultural, and industrial activities [1–3]. While water
demand enormously increases, the availability of freshwater is being exhausted because
of an escalation in pollution, thus causing water scarcity for modern civilization [4,5].
Rapid industrialization, haphazard urbanization, and various anthropogenic activities
along with improper waste disposal direct to such an increase in wastewater. Dyes and
heavy metals are the two most typical contaminants found in industrial wastewater, which
catastrophically affects sustainable ecosystems [6–9]. The existence of dyes, even at low
concentrations, limits sunlight penetration into water that causes a significant reduction of
dissolved oxygen, creating critical health risks to the aquatic living bodies. In many cases,
dyes induce anaerobic digestion to produce different carcinogenic compounds, which can
enter the food chain via aquatic organisms [10]. However, the amount of dye disposal
into different water sources is still significant. For instance, over 7 × 105 tons of different
reactive dyes are produced annually, whereas about 5–10% of these dyes end up in the
Gels 2021, 7, 30. https://doi.org/10.3390/gels7010030 https://www.mdpi.com/journal/gels

Gels 2021, 7, 30 2 of 28
industrial effluents [11]. Heavy metals, on the other hand, cause serious threats to human
health because of their high carcinogenic and toxic nature. For example, a chronic intake
of arsenic (As) can cause severe diseases such as kidney, prostate, bladder, or liver cancer.
Chromium (Cr) is another highly toxic metal that unpleasantly affects human bodies as
well as various aquatic species [12]. The hazardous nature of both dyes and heavy metals
to public health and the natural ecosystem highlights the necessity of efficient treatment
of industrial wastewater. Table 1 summarizes the adverse impacts of heavy metals and
synthetic dyes on human health and the environment.
Table 1. Negative impacts of heavy metals and synthetic dyes on human health.
Pollutants Potential Negative Impact on Human Health Ref.
Heavy
metals
Zn Stomach cramps, dermal irritations, vomiting, nausea, and anemia [13]
Cu Severe toxicological complications such as vomiting, cramps, convulsions, and even death [14]
Ni Acute lung, kidney and gastrointestinal pain, pulmonary fibrosis, and skin dermatitis [15]
Hg Pulmonary, kidney, and chest pain, dyspnea impairments [16]
Co
Paralysis, asthma, pneumonia, diarrhea, lung irritations, weight loss, vomiting, nausea,
damage thyroid hormone and liver
[17–19]
Cd Renal dysfunction and even death [13]
Pb
Fauilure of kidney, liver, reproductive system, basic cellular processes, brain function, and
even the central nervous system of the human body
[20–22]
Cr Destruction of human metabolism, food chain disruption, skin irritation, lung carcinoma [23,24]
Synthetic
dyes
Azo dyes
DNA destruction, carcinogenic and mutagenic, skin irritation, hypertension, tongue and
larynx distress, blindness, acute tubular necrosis, gastritis, respiratory distress, liver issues,
bladder cancer, neurosensory harm
[25–30]
Reactive dyes
Respiratory diseases, asthma, coughing, wheezing, sneezing, watery eyes, itching, respiratory
sensitization, poor immune system, deteriorate the water quality and damages to water
bodies, ecosystem and biological cycle
[31–33]
Vat dyes Skin irritation [33]
Sulfur dyes Unpleasant odor, carcinogenic, skin irritation, allergic dermatitis, mutations [34,35]
Disperse dyes Bioaccumulation in nature, genotoxic in mammalian assays, mutagenic [36]
Adsorption is the most common and effective method used for wastewater treatment
because it is convenient, simple, less expensive, and has no harmful by-products [37–40].
Activated carbon as an adsorbent is highly efficient that is often used commercially to
remove pollutants from wastewater. However, the excessive cost and complicated re-
generation process limit the widespread utilization of activated carbon for wastewater
treatment [41]. Although recent studies have discovered numerous adsorbents for the
removal of dyes and heavy metals from industrial effluent, the use of these adsorbents
for bulk treatment of wastewater is still challenging [42,43]. HGs are three-dimensional
polymer networks, have attracted incredible attention nowadays to eliminate contaminants
from waste streams due to their high removal efficiency [44–47]. HGs are highly porous
and comprised of numerous hydrophilic functional groups (e.g., –OH, −COOH, −NH2,
−SO3H, −CONH2, etc.) that enable the adsorption and retention of a large volume of
water during the treatment process and eventually cause up to the complete removal
and recovery of aqueous dyes and heavy metals [48,49]. However, most of the existing
HGs are derived from petrochemicals, which are neither renewable nor biodegradable.
CBHs derived from cellulose, the most abundant polymer in nature, are superabsorbent,
durable, biodegradable, biocompatible, and non-toxic [50]. Recently, researchers have re-
ported almost complete removal of dyes and heavy metals from wastewater using different
CBHs. For instance, Zhou et al. [51] achieved ≈90% of Pb+2 removal efficiency (adsorption
capacity of 171 mg/g) within four hours of the adsorption process using carboxylated

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cellulose nanofibril-based hydrogels. Such high adsorption values were also obtained for
Cu2+ (182–230 mg/g), Ni2+ (200 mg/g), and Hg2+ (140 mg/g) using different types of
CBHs [51–54] Recently, Deng et al. [55] successfully removed almost 100% Congo Red dyes
with 166.1 mg/g saturation adsorption using chitosan and cellulose. Numerous scopes
of using CBHs still exist, since the modification of HG functional groups enhances its
adsorption effectiveness.
While numerous research works have been published using CBHs to treat wastewater
streams, the lack of review in this specific area is the motivation of the present work. Table 2
demonstrates the gap in the literature based on the recently published review articles that
are closely aligned to the proposed topic. A recently published book chapter has covered
a few parts of the topic; however, it lacks the critical content, such as different adsorp-
tion mechanisms required to develop a comprehensive understanding of the topic [56].
This review is systematically designed to include critical information on the topic such
as the development of CBHs for the dye and heavy metal adsorption from industrial
effluent, adsorption mechanism, and factors affecting the adsorption capacity along with
future outlook.
Table 2. Recently published review articles on wastewater treatment using hydrogels.
Types of Hydrogel Types of Pollutants Major Contents Ref.
Synthetic and natural hydrogels Heavy metal Selectivity, efficiency, and reusability [57]
Synthetic and natural hydrogels Heavy metal
Factors of adsorption and detection of
metal
[58]
Synthetic and natural hydrogels
Aqueous pollutants including
dye, heavy metals, and anions
Adsorption kinetics, regeneration, and
reusability
[49]
Synthetic and natural hydrogels
Dye, heavy metal, radioactive
materials, pesticides
Adoption properties, kinetics, isotherm,
mechanism, factors, recycling, and
recovery
[59]
Synthetic and natural hydrogels Dye and metal
Synthesis, mechanism, modification of
adsorbents, and kinetics
[60]
Acrylic-based hydrogels Dye and heavy metals
Preparation and adsorption properties of
different acrylic-based HG
[61]
Hybrid hydrogels
Metal, radionuclides, anions,
acid phenol ammonium
Adsorption properties [62]
Composite hydrogels Dye
Adsorption properties of different types of
composite hydrogels
[63]
Cellulose-based hydrogel Dye and metal
Preparation, adsorption mechanisms, and
factors affecting adsorption capacity
This paper
2. Preparation of CBH
A plethora of challenges generated by the excessive use of fossil resources and non-
biodegradable materials result in shifting the attention of researchers toward renewable
and environmentally safe materials. At this juncture, biopolymers are used in different
areas such as agriculture, food packaging, biomedical applications, and wastewater treat-
ment [55,64–70]. Similarly, when it comes to the removal of dyes and heavy metals by
adsorption process, biopolymer-based hydrogels such as CBHs are suitable due to their
better functionality, good solubility in organic solvents, enhanced surface area, abundancy,
low-cost, better adsorption capacity, biodegradability, and ease of fabrication and recycla-
bility. Additionally, the excellent hydrophilicity makes the CBHs a promising adsorbent for
wastewater treatment [71–73]. However, the performance of an adsorbent for the removal
of pollutants from wastewater is highly selective to the physical and chemical properties
of the adsorbent materials [47]. Therefore, the first and foremost step for an effective

Gels 2021, 7, 30 4 of 28
adsorption process is to synthesize a suitable CBH adsorbent having high adsorbability to
the dyes and heavy metal ions present in wastewater streams.
CBHs are typically derived from cellulose (i.e., native/pure cellulose and bacterial
cellulose), its derivatives (ether derivatives: methylcellulose, ethylcellulose, hydroxyethyl
methylcellulose, hydroxypropyl cellulose, carboxymethyl cellulose (CMC), etc.; ester
derivatives: acetate trimellitate, acetate phthalate, hydroxypropyl methyl phthalate, hy-
droxypropyl methyl phthalate acetate succinate, etc.), and/or its composites (such as
polyelectrolyte complex, interpenetrating polymer networks and blending with other poly-
mers) [48,74]. One of the greatest challenges of HG synthesis from these base materials is
dissolution in a common solvent; while some of the cellulose derivatives are water-soluble,
native/pure cellulose is hardly soluble in most common organic and inorganic (e.g., water)
solvents. Consequently, a suitable solvent is a prerequisite for CBH synthesis that includes
alkali/urea (or thiourea), LiCl/dimethylacetamide, N-methyl morpholine-N-oxide, and
ionic liquids.
A stable 3D network structure of HG that is critical to hold a huge amount of water
in their interstitials is achieved by crosslinking the constituting polymer chains [74,75].
Cellulose has hydrophilic functional groups, such as hydroxyl (−OH), that enable them
to form both physical crosslinking (electrostatic interaction) and chemical crosslinking
(covalent interaction using crosslinker). Depending on the materials used and modes of
fabrication, different types of interactions are observed in CBHs, such as (i) small cations–
cellulose (electrostatic interaction), (ii) polycations–cellulose (electrostatic interaction), (iii)
polymer–cellulose (H-bond or hydrophobic interaction), (iv) self-assembly, (v) coordination
complex crosslinking, and (vi) covalent crosslinking [76]. Figure 1A illustrates different
types of interactions between cellulose molecules and other polymers or small molecules
within a hydrogel matrix. All these interactions lead to the formation of the crosslinked
structure of the hydrogels that prevents the complete destruction and dissolution of the
CBHs during swelling. Cellulose-based hydrogels can be designed in a variety of physical
shapes, including spherical, cylindrical, bead, blocks, microparticles, nanoparticles, and
films by diverse methods of fabrication.
Figure 2 shows different pathways of synthesis of CBHs. Supplementary Materials
Tables S1 and S2 include recent works on heavy metal and dye removal using CBHs along
with their adsorption capacities.
2.1. Physical Path of Crosslinking
However, physical crosslinking process held by weak connections such as hydrogen
bonds (H-bond), ionic interactions, hydrophobic interactions, π–π interactions, and van der
Waals forces [72,77–83] is often more favorable for ecofriendly and non-toxic HG synthesis,
as it does not involve use of chemical-based crosslinking agents [84] Although CBHs
developed by the physical crosslinking process suffer from poor mechanical properties,
they are widely used for adsorption purposes because of their high porosity having a
higher chance to adsorb more pollutants, low sensitivity to pH, ease of regeneration (being
reversible process), and no reduction in adsorption capacity due to potential reaction
with a crosslinker as in the chemical process [60,85,86]. These HGs are prepared in the
following methods.
2.1.1. Freeze–Thaw
One of the physical methods to prepare HGs is crystallization by the freeze−thaw
technique [83]. In the freeze–thaw technique, crystallization occurs by freezing bulk
solvents or low molecular solutes that increase the concentration of polymer by minimizing
the chain space in the polymer and enable the chains to align and connect to each other
to form a network structure (illustrated in Figure 1B) [75,87]. Freeze−thaw cycles allow a
porous structure to be created in HGs because of the space left from the melting crystals
at thawing stages [87,88]. The mechanical properties of the freeze−thawed HGs can be

Gels 2021, 7, 30 5 of 28
modulated by altering the concentration of polymers, the number of freeze−thaw cycles,
the freezing and thawing time, and the freezing temperature [89].
HGs prepared by the freeze−thaw process show better elastic properties than that of
the HGs prepared by chemical methods and hence drawing large-scale attention around
the world [90]. Bio-compatible and non-toxic polysaccharide-based (such as cellulose,
dextran, pullulan, and carboxymethyl curdlan, CMC, etc.) HGs are developed following
this process [91]. For example, PVA (poly vinyl alcohol)/CMC HGs are prepared by
freeze−thaw processes and applied to adsorb heavy metal ions including Ag+, Ni2+, Cu2+,
and Zn2+ [87].
Figure 1. Illustration of (A) interactions in cellulose-based hydrogels in different systems—physical crosslinking (i–v)
and chemical crosslinking (vi), and (B) an example of physical crosslinking (freeze–thaw). (A) (i) Electrostatic interaction
between small cations and cellulose chain; (ii) electrostatic interaction between opposite charges of polycation molecule and
cellulose chain; (iii) H-bond or hydrophobic interaction between polymer molecule and cellulose chain; (iv) self-assembly
of cellulose molecules to fold into scaffolds by weak non-covalent bonding mechanisms—including hydrogen bonds, van
der Waals forces, and hydrophobic interactions; (v) coordination complex crosslinking between multivalent metal ions
and cellulose chain; and (vi) covalent crosslinking among functional moieties of cellulose chains and/or polymer chains,
sometimes with the help of crosslinkers. (B) Fabrication of hydrogels through physical crosslinking by freeze–thaw method.
2.1.2. Self-Assembling
The idea of using self-assembled CBHs preparation has grabbed the attention of
researchers, as it does not need any crosslinkers to prepare. Self-assembling HGs are
developed when the constituting monomers in the form of fibrils are assembled via non-
covalent interactions (van der Waals forces, electrostatic interactions, hydrogen bonding,
and π–π stacking interactions) followed by the entanglement of the fibrils into a robust
network [92]. The mechanical properties of these HGs can be tuned by changing the
concentration of the constituting monomers. Cationic guar gum (CGG) and TEMPO (2,2,6,6-

Gels 2021, 7, 30 6 of 28
tetramethylpiperidine-1-oxyl)-oxidized cellulose nanofibers (TOCN) can instantaneously
form HGs as soon as they come in contact with each other. Formed HGs can adsorb metal
ions as well as dyes from wastewater. They can also be used to treat oily wastewater by
coating filter papers in a layer-by-layer deposition process [71].
Figure 2. Physical and chemical paths of cellulose-based hydrogel (CBH) synthesis.
2.1.3. Instantaneous Gelation
The instantaneous gelation method is another way to develop HG instantly following
the one-step method [93,94]. Magnetic chitosan-based HG beads (m-CS/PVA/CCNFs),
consisting of carboxylated cellulose nanofibrils (CCNFs), amine-functionalized magnetite
nanoparticles, and poly (vinyl alcohol) (PVA) blended chitosan, have been prepared by
the instantaneous gelation method as adsorbents for Pb(II). The prepared HG beads can
be persevered in distilled water for future use and recovered with the aid of a magnet if
required [51].
2.1.4. Reconstitution
Cellulose-based composite HGs are prepared by the reconstitution method that often
uses ionic liquid as a solvent. Here, the composite HG is held by a strong intermolecular H-
bond that eventually contributes to the tensile strength of the developed HG. For instance,
biodegradable collagen/cellulose HG beads (CCHBs) have been prepared by reconstitution
from a 1-butyl-3-methylimidazolium chloride ([C4mim] Cl) solution that can potentially be
used to adsorb both dyes and heavy metals [95].
2.1.5. Inverse Emulsion Technique
Water-in-oil emulsion refers to a phenomenon that occurs when water droplets are
dispersed in oil (the continuous phase: paraffin oil) by using a suitable stabilizing agent
(such as non-ionic surfactant Triton X-100), and then the system undergoes phase inversion
in a coagulation bath to leach out the droplets and precipitates the porous film. This
mechanism is widely known as the inverse emulsion technique. Partially hydrolyzed
polyacrylamide grafted Arabic gum (AG-g-PAM/PAA)-based HG has been prepared by
the inverse emulsion technique that can be used for dye and metal ion adsorption. This
HG showed the adsorption of Methylene Blue (MB) with a capacity of 200 mg/g from an
aqueous solution. The results revealed an ability of the novel porous HG to adsorb 99% of
dye in only 10 min [96]. In addition to that, HGs based on sodium carboxymethyl cellulose
and acrylic acid were prepared by inverse emulsion polymerization using potassium

Gels 2021, 7, 30 7 of 28
persulfate as an initiator and N,N0-methylenebisacrylamide as a crosslinker. The maximum
swelling capacity for the HGs was 544.95 g/g in distilled water and 44.0 g/g in 0.9% w/v
NaCl solution [97].
2.1.6. Ionotropic Gelation
The ionotropic gelation (IG) technique allows the production of nanoparticles and mi-
croparticles by electrostatic interactions between two ionic species under certain conditions
where at least one of the species should be a polymer [98]. CBH beads are successfully
synthesized by the ionotropic gelation of sodium alginate (SA) and hydroxypropyl cel-
lulose (HPC) solutions with varying ratios (0:50, 75:25, and 100:0) followed by extrusion
through a syringe to form HG beads. The adsorption property of the produced bead is
largely influenced by the concentrations of SA and HPC. The beads showed 47.72 mg/g
adsorption capacity and a 95.45% adsorption percentage of Pb2+ [99]. Cellulose nanocrys-
tals and alginate-based HG beads are also synthesized by the ionotropic gelation that can
be used for the adsorption of organic dyes [100].
2.2. Chemical Path of Crosslinking
Chemical crosslinking is preferred when high mechanical strength is required through
introducing a chemical crosslinking agent forming strong molecular bonds such as via
covalent and electrostatic interactions. However, the degree of crystallinity negatively
impacts the adsorption capacity and swelling ratio due to the reduced pore size and
rigidity of polymer chains [61]. In addition, crosslinking agents and the higher crosslinking
density engage some binding sites of the adsorbent; therefore, there should be a balance
between maintaining the required mechanical strength and maximizing the adsorption
capacity [60,61]. The chemical path of crosslinking can be achieved in the following ways.
2.2.1. Crosslinking by Chemical Reaction
In chemical crosslinking, the bond is formed between the crosslinking agent and
polymer or among the functional groups of polymer molecules. A polymer having hydroxyl
(such as cellulose and its derivatives) and amine (such as chitosan, proteins) groups are
connected with crosslinker having an aldehyde group (aldol formation) via covalent
bond [75]. HG formation for the polymers with –OH groups requires specific conditions
such as lower pH, high temperature, and methanol as a quencher, whereas protein-based
HGs can be formed in normal conditions [61,101–103]. A typical crosslinker for CBHs
includes glutaraldehyde and epichlorohydrin, formaldehyde, acetaldehyde due to their
availability, and cost-effectiveness [61,75,104]. For instance, environment-friendly CBH
beads have been produced from CMC by using an epichlorohydrin (ECH) crosslinker
in an inverse suspension (fluid wax) crosslinking mechanism, where ether linkage is
developed between ECH and CMC contributing to high porosity in the structure of HG.
More specifically, the porous structure of the HG is attributed to the presence of numerous
carboxylate anions (–COO−) in a network of HG beads, which not only helps expand
the HG network but also increases the size and amount of pore enabling more metal
ion to anchor [105]. Ge et al. also prepared a high-performance composite hydrogel
(cellulose/poly-ethylene imine (PEI)) by grafting hyperbranched PEI onto cellulose chains
using an ECH crosslinker [106]. The hydrogels showed excellent Cu2+ ion removal capacity
(ca. 285.7 mg/g) and good stability over a wide range of pHs and temperatures due to
the chemical crosslinking facilitated by ECH. Similarly, in the presence of a crosslinker,
condensation and additional reactions occur between polymers having amine and ester
moieties, and a crosslinker, causing Schiff base formation [107].
2.2.2. Crosslinking by Polymerization
A large number of CBHs are synthesized by the chain growth polymerization process,
which is accomplished in three steps including initiation, propagation, and termination [49].
Among different types of polymerization processes, free radical is characterized by a faster

Gels 2021, 7, 30 8 of 28
synthesis process, ease of implementation under various reaction conditions, wide-ranging
temperatures, and low costs, and it is widely followed for HG preparation [75,108]. Here,
for the initiation step, a free radical is generated from the initiator such as potassium
persulphate (KPS), tetramethylene-diamine (TEMED), ammonium persulfate (APS), etc.,
in the presence of some conditions in term of light, pressure, temperature, and radia-
tions. In propagation, polymer chain growth occurs, and the crosslinker reacts with the
growing polymer chain, randomly leading to forming a 3D network structure, followed
by the termination of the polymerization via combination or disproportion [49,60,61].
Cellulose-graft-acrylic acid (C-g-AA) HGs are prepared via free-radical polymerization in
85% phosphoric acid solution in the presence of 50 mg ammonium persulfate (APS) as an
initiator and 10 mg N,N0-methylene bisacrylamide (MBA) as a crosslinker. Acrylic acid
acts as a grafting monomer and can bind heavy metal ions and dyes through its carboxyl
groups. The mechanism behind the HG preparation is the extraction of hydrogen atoms
from the cellulose molecules to produce cellulose macro-free radicals on the cellulose
chains using peroxodisulphate (S2O8
2–) [52]. In another study, a similar type of hydrogel
was prepared from cellulose and CMC separately or in a mixture of both of them by poly-
merization with partially neutralized AA [109]. The polymerization was initiated by KPS,
and vinylsulfone (VS), glutaraldehyde, MBA, and ECH were used as crosslinkers. The
hydrogel was successfully used to remove and recover heavy metals such as Cu(II) from
wastewater. The addition of a modifier such as tannic acid (TA) to cellulose-based hydrogel
can be helpful to attain a homogeneous pore structure of the hydrogel that improves the
adsorption performance. Ning et al. [110] synthesized an HEC-co-p(AA-AM)/TA (HEC:
hydroxyethyl cellulose, and AM: acrylamide) hydrogel by the grafting of AA and AM onto
HEC, followed by modification with TA. The hydrogel showed excellent MB adsorption
performance (ca. 3438.27 mg/g) with high reusability. In a recent study, a novel fluorescent
lignin-based hydrogel with cellulose nanofibers and carbon dots (CDs) was synthesized
by free radiacal polymerization [111]. The hydrogel demonstrated 3D porous structures
that provided many active sites and ion transport channels, thereby improving the ad-
sorption performance for hexavalent chromium(Cr(VI)) (maximum adsorption capacity
599.9 mg/g).
2.2.3. Crosslinking by Radiation
It is also a polymerization process, where no chemical-based crosslinker or catalyst is
used. Instead of a chemical crosslinker as in the usual polymerization process mentioned
above, it induces different types of radiations (such as gamma, electron beams, microwave,
and ultraviolet radiations) to crosslink the polymer chains. Therefore, it is an environmen-
tally benign synthesis process involving zero waste generation [60,61]. CMC-Na is a widely
used derivative of cellulose, which is synthesized by gamma irradiation [53].
3. Adsorption Mechanism
A comprehensive understanding of the adsorption process along with the removal
mechanism of several pollutants on respective CBHs is very indispensable for further
modification of CBHs to improve their adsorption performance. The adsorption by CBHs
typically occurs through different types of interactions, which are extensively dependent
on the functional groups present in the HG, adsorbent properties, the chemical composition
of pollutants, and experimental parameters (i.e., initial pollutant concentration, solution
pH, temperature, the coexistence of metal ions, etc.) [60]. The most common adsorption
mechanism for the removal of dyes and heavy metals by CBHs is electrostatic interactions;
however, combinations of other interactions along with electrostatic interaction are also
reported in many adsorption processes [105,112–114]. The different adsorption mechanisms
of dyes and heavy metals by CBHs are shown in Figures 3 and 4.

Gels 2021, 7, 30 9 of 28
Figure 3. Adsorption mechanism of dyes and heavy metals by CBHs.
3.1. Electrostatic Interactions
Electrostatic interaction comprises the interaction between charged modules; attrac-
tive and repulsive interaction occurs when molecules are oppositely-charged (cation–anion
interactions) and similarly-charged (cation–cation or anion–anion interactions), respec-
tively [115]. To remove ionic contaminants electrostatically by CBHs, the adsorbent surface
must oppositely be charged to the respective ions that need to be adsorbed. Based on
the pollutant nature and chemical properties, the CBHs are synthesized with specific
functional groups, which are capable of producing the opposite charge corresponding to
target ions [60]. Additionally, the formation of charged species on the adsorbent surface
intensely depends on the pH of the solution [116]. pHPZC represents the pH of the solution
when no charge species exist on the adsorbent surface [117–119]. At pH pHPZC, func-
tional groups, such as –COOH, –OH, and –H3PO4 are deprotonated due to an excessive
concentration of OH− in the aqueous solution that creates anions (such as –COO−,–O−,
–PO4
3–, etc.) on the adsorbent surface, resulting in attractive interactions between cationic
contaminants and the anionic adsorbent surface (Figure 4). In contrast, the adsorbent
surface is positively charged pH pHPZC due to the protonation of functional groups
(i.e., –NH2, –SH, etc.) on the adsorbent surface as a consequence of an increase in H+
concentration in the solution. Here, a recent study also found the highest electrostatic
interactions between a high-capacity HG with –NH2 groups and an anionic dye (Acid
Black 1) at low pH conditions, which was decreased with rising pH and reached the least
interactions for very high pH condition [112]. In another study, CMC adsorbed the utmost
amount of Pb2+, Cu2+, and Ni2+ from aqueous solution through electrostatic interactions
at higher pH [105]. Similar electrostatic nature was perceived for the removal of MB (a
cationic dye) by chemi-mechanical pretreated cellulose-based superabsorbent hydrogel [97].
Hu et al. [120] also observed electrostatic adsorption with complexation between –OH and
–COOH functional groups of sodium alginate–CMC–cellulose gel beads for the removal
of Pb2+.

Gels 2021, 7, 30 10 of 28
Figure 4. Adsorbent–adsorbate interaction mechanisms for the decontamination of wastewater by CBHs.
3.2. Ion Exchange
Ion exchange refers to an exchange of ions between a liquid (wastewater) and an
insoluble solid (adsorbent). Unwanted dissolved ions (cations or anions) in an aqueous
solution are removed and replaced with ions of the same charge on the adsorbent surface.
In a perfect ion-exchange process, the number of ions released from the adsorbent sur-
face is equivalent to the number of ions adsorbed by the adsorbent molecules [121]. Ion
exchange is a very convenient and efficient tool especially for the removal of hazardous
pollutants, such as dyes and heavy metals from wastewater. This process decreases the
degree of hazardous load by transforming pollutants into a shape in which they can be
recycled, leaving behind less harmful elements in their place or enable ultimate discharge
by decreasing the hydraulic flow of the stream carrying toxic elements. In addition, the

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ion-exchange process has the capability to discrete as well as distillate contaminants [122].
Similar to electrostatic interactions, ion-exchange mechanism also shows a strong depen-
dency on the pH of the solution. At pH pHPZC, the functional groups of adsorbent
are positively charged because of an increase in H+ concentration, resulting in cations
exchange. On the other hand, functional groups are negatively charged when pH pHPZC,
which causes anions exchange (Figure 4) [123]. Zhou et al. [124] observed ion exchange and
chelation between positively charged ions (Cd2+, Ni2+, and Pb2+) and ionized/non-ionized
carboxylic groups within the HG during the removal of these metal ions from aqueous
solutions using cellulose–graft–acrylic acid hydrogel at pH 2.5–6.0, since –COOH groups
in the HG surface are protonated at lower pH, which replaced metal ions with H+ from
–COOH groups. In another investigation, Ca2+ ions replaced cations from cellulose–graft–
polyacrylamide/hydroxyapatite composite HG and attached to the hydroxyapatite surface
through an ion-exchange mechanism [125]. Xiong et al. [126] developed a self-cleaning
hybrid (cellulose–titanate) hydrogel microsphere by a simple sol–gel process that exhibits
an excellent ability to remove heavy metal from oily wastewater. The strong physical
and chemical interaction between titanate nanotubes (TNTs) and cellulose fibers helped
inherit and integrate the intrinsic properties of both titanate and cellulose hydrogels. At
first, the heavy metal ions (Cu(II)) were adsorbed on the inside of the hydrogel under the
electrostatic interaction. Then, through ion exchange, Cu(II) ions were deeply trapped in
the layer structure of TNTs. Thus, under the synergistic effects of physical and chemical
adsorption, the hydrogel revealed excellent adsorption properties for heavy metal ions.
Similar ion-exchange mechanisms have been found for the removal of dyes from aqueous
solutions [127–129].
3.3. Hydrogen Bonding
H-bonding is a distinct form of dipole–dipole interaction that results from the elec-
trostatic attractive force between a positively charged H-atom and a more electronegative
atom (i.e., N, O, F, etc.) or group, which are covalently bonded [130]. During the treatment
of dye-containing wastewater, functional groups having oxygen (i.e., –COOH, –OH) in
the adsorbent molecules participate in H-bonding with pollutants (dyes) [131,132]. The
adsorption of MB on a CBH (synthesized by modification of cellulose and acrylic acid)
showed such interaction between electronegative N-atom in MB structure and H atom
in –COOH and –OH groups of HG (Figure 4) [133]. Recently, Sekine et al. [70] developed
eco-friendly CMC nanofiber HGs, which were used to remove numerous chemical dyes
through hydrogen bonding, electrostatic interactions, and hydrophobic interactions be-
tween the functional groups of dye and adsorbent molecules. Lie et al. [46] extensively
explained the H-bonding interactions between the sulfur (S) atoms in both anionic (Acid
Blue 93) and cationic (Methylene Blue) dyes and the H atoms in a CBH material along with
the graphical presentation.
3.4. Hydrophobic Interactions
Hydrophobic interaction defines the interaction between hydrophobes and water
molecules. Hydrophobes are non-polar compounds, which are composed of long chains
of carbon that cannot interact with water molecules due to weak van der Waals attractive
forces [134]. In addition, low water-soluble elements show a high tendency to be attracted
to hydrophobes. Therefore, during wastewater treatment, hydrophobic interactions are
formed to remove non-polar pollutants (i.e., pigments, disperse dyes, organic compounds,
etc.) from aqueous solution (Figure 4) [135]. Li et al. [136] demonstrated both electrostatic
and hydrophobic interactions between MB dye and functional groups (–SH and –OH)
present in thiol-modified CMC/L-cysteine HG. Hydrophobic interactions in CBHs offer
extensive opportunities in HG engineering because of their roles in enriching mechani-
cal properties [137,138]. Furthermore, the domain of hydrophobes provides a physical
crosslinking point with optimal mechanical stiffness during the initiation of chemical reac-
tions. The reaction continues with the formation of interactions between crosslinking points

Gels 2021, 7, 30 12 of 28
and other polymeric chains until macromolecular three-dimensional polymer networks
are formed [139]. Lazzari et al. [140] recently showed that hydrophobic interaction is one
of the main driving forces to adsorb insoluble organic pollutants into cellulose cryogels.
However, CBH is often modified with both hydrophilic and hydrophobic functional groups
to remove pollutants from aqueous solutions. The hydrophilic part of functional groups at-
tracts soluble ionic pollutants through either electrostatic, H-bonding, and/or ion-exchange
interactions, while the hydrophobic part contributes to the adsorption of water-insoluble
contaminants [141,142].
3.5. Coordination Interactions
The coordination interaction refers to a covalent bond wherein both electrons are
shared by a single atom. In the removal of cations (heavy metal ions and/or cationic dyes)
from wastewater through a coordination mechanism, cations attract atoms in functional
groups that have lone pair electrons (i.e., O and N) in outer orbitals, resulting in the
adsorption of cations on the adsorbate surface. Coordination interactions can also happen
along with other interactions including ion-exchange, electrostatic interactions (Table 3).
For example, at low pH, H+ ions from the adsorbent surface have been replaced with metal
ions present in the solution, and these metal ions have a high affinity to negative electrons.
Typically, different metal ions as well as N and O atoms in dye molecules are adsorbed
on ion-exchanged functional groups (i.e., –OH) via coordination bonds (Figure 4). One
such experiment was recently conducted by Jayabrata Maity and Samit Kumar Ray (2017),
where they observed the combined effects of coordination and electrostatic interactions
during the removal of Cu2+ using sugar cane bagasse cellulose and gelatin-based composite
hydrogels. Cu2+ ions formed coordination bonds with N or O atoms, which were sourced
from –NH2 and –OH functional groups, respectively [143]. Tang et al. [54] also observed
coordination interactions between metal ions and the O atoms (from –OH group) during
the removal of Hg2+, Pb2+, and Cu2+ using chitin/cellulose composite (3:1) adsorbent.
The adsorption mechanism of chitosan/cellulose composite adsorbent for the removal
of Congo Red (CR) dye revealed electron sharing (coordination interaction) and transfer
(electrostatic adsorption) between the adsorbent and adsorbate molecules [55].

Gels 2021, 7, 30 13 of 28
Table 3. Proposed removal mechanism of contaminants by CBHs.
CBHs Synthesis Method Pollutants Proposed Mechanisms Ref.
Cellulose–bentonite porous composite Crosslinking Azo dye Electrostatic interaction [144]
Carboxymethyl cellulose HG beads Inverse suspension crosslinking Cu2+, Ni2+, Pb2+ Electrostatic and coordination interactions [105]
Chemi-mechanical pretreated cellulose-based
superabsorbent HG
Modification of cellulose and acryloyl
chlorides
Methylene Blue Electrostatic interactions and H-bonding [133]
Superadsorbent cellulose–graft–acrylic acid Free-radical polymerization Methylene Blue Electrostatic interactions [145]
Cyanoethyl cellulose Ionic xanthate graft polymerization Cu2+ Electrostatic interactions [146]
Carboxymethyl cellulose-based magnetic
superabsorbent
Simultaneous magnetic ion oxides
nanoparticles and superabsorbent
formation
Crystal violet Electrostatic interactions [147]
Cellulose–graft–
polyacrylamide/hydroxyapatite composite
HG
Suspension polymerization Cu2+ Ion exchange [125]
Sugar cane bagasse cellulose and
gelatin-based composite HGs
Crosslinking Cu2+ Electrostatic and coordination interactions [143]
Carboxylated cellulose nanocrystal-sodium
alginate HG beads
Crosslinking Pb2+ Complexation and electrostatic interactions [120]
Carboxylated cellulose nanofibrils-filled
magnetic chitosan HG beads
Instantaneous gelation Pb2+ Electrostatic adosption [51]
Carboxymethyl cellulose–graft poly(acrylic
acid)/monmorilonite HG composite
Graft polymerization Pb2+, Zn2+ Ion exchange and coordination interactions [148]
Hydroxypropyl cellulose/molybdenum
disulfide composite HGs
Graft polymerization Methylene Blue Electrostatic interactions [149]
Cellulose–graft–acrylic acid HGs Grifting reaction mechanism Cd2+, Ni2+, Pb2+ Electrostatic interactions and ion exchange [124]
TEMPO-oxidized cellulose HGs Nitroxy radical catalyzed oxidation Zn2+, Fe3+, Cd2+, Cs+ Electrostatic interactions and ion exchange [150]
Chitin/cellulose composite HGs Freezing/thawing Hg2+, Pb2+, and Cu2+ Electrostatic and coordination interactions [54]
Cellulose-based bio-adsorbent Graft copolymerization Acid Blue, Methylene Blue Electrostatic interactions and H-bonding [46]
Carboxymethyl chitosan/poly (acrylonitrile)
HGs
Crosslinking Cu2+, Cd2+, and Co2+ Electrostatic interactions [151]

Gels 2021, 7, 30 14 of 28
Table 3. Cont.
CBHs Synthesis Method Pollutants Proposed Mechanisms Ref.
Chitogen/Cellulose HGs Freeze-dried Congo Red Electrostatic and coordination interactions [55,152]
Carboxymethyl cellulose structured
nano-adsorbent
Sol–gel method Methyl Violet Electrostatic and π–π interactions [153]
Nanocomposite HG Graft polymerization Crystal Violet Electrostatic interactions, H-bonding [154]
CMC–acrylamide–graphene oxide HGs Radical polymerization Acid Blue 133 Electrostatic interactions [81]
Lignocellulose-g-poly(acrylic
acid)/montmorillonite 3D crosslinked
polymeric netwrok HGs
Copolymerization Methylene Blue Electrostatic interactions [155]
Carboxymethyl Cellulose gel γ-irradiation Cu2+ Chelation (coordination interactions) [53]
CMC-acrylic acid adsorbent Graft polymerization
Methyl Orange, Disperse Blue
2BLN, and Malachite Green
Chloride
Electrostaic interactions [152]
CMC/2-acrylamido-2-methyl propane
sulfonic acid HGs
Copolymerization and crosslinking Co2+, Cu2+, Mn2+, Fe3+ Electrostatic and chelating interactions [156]

Gels 2021, 7, 30 15 of 28
3.6. π–π Interactions
π–π interaction is a non-covalent interaction between adsorbent and adsorbate molecules
in an aqueous solution. Numerous chemical properties, such as chemical bonding, boiling
points, molecular and biomolecular crystallography, the structure of π-adjacent molecules,
etc. are widely affected by π–π interactions [157]. In typical π–π interaction, at least one of
the molecules contains a π electron-rich or a deficient group in the structure of benzene or
other aromatic rings that causes interactions in an aqueous medium. The π–π interaction
significantly depends on the functional groups present on both the adsorbate and adsorbent
surface and medium of the solution (pH) [158]. Based on these functional groups coupled
with the pH of the solution, the adsorbate and adsorbent molecules act as an electron-donor
or electron-acceptor, resulting in forming various π–π interactions (electron-donor-acceptor,
electron-acceptor-acceptor, and electron-donor-donor) [159]. These types of interactions
are usually found during the adsorption of organic pollutants and dyes onto graphene-,
graphene oxide-, or carbon-based HGs. Sharma et al. [153] attained an adsorption capacity
of 96.43 mg/g for Methyl Violet (dye) using a CMC-structured nano adsorbent. π–π
stacking with electrostatic interactions were reported as the potential causes for such
high adsorption capabilities where contaminants donated π-electrons to the adsorbent
molecules. Chen et al. [160] also reported that π–π stacking is the primary driving force
in the removal of heavy metal ions (Cu2+, Zn2+, Fe3+, and Pb2+) from wastewater using
GO/cellulose HG. The –COOH and –OH functional groups on the adsorbent surface
(introduced from GO) made π–π interactions with adjacent metal ions. The role of GO in
the HG was to enhance mechanical strength as well as the adsorption capability of porous
GO/cellulose HGs. Another type of GO/cellulose HG with high mechanical and thermal
stabilities was prepared to remove contaminants from waste solution [161]. In another
study, Yan et al. [162] prepared a self-healing HG with great mechanical strength based on
cellulose-derived co-polydopamine@Pd nanoparticles for the reduction of contaminant
dye in wastewater. They attained 95% removal of both anionic and cationic dyes from
wastewater through π–π interactions, hydrogen bonding, and coordination interactions
without a significant decrease in the performance or integrity of the HG structure, even
though the water molecules continuously weaken to van der Waals interaction to decrease
the mechanical properties and stability of HG [163–165]. For carboxymethyl cellulose
sodium (CMCNa)/graphene oxide (GO) hydrogel microparticles, Liu et al. [166] suggested
that the adsorption mechanism for dyes were due to both electrostatic and π–π interactions,
while those for heavy metals were the synergistic effect of electrostatic interactions, surface
complexation, and ion exchange.
4. Factors Affecting the Adsorption Capacity of CBHs
4.1. Crosslink Density
Crosslink density refers to the density of chains or segments that attach two or
more parts of the polymer network, instead of the density of crosslink junctures. The
adsorption capacity of CBHs is highly affected by the crosslink density of polymeric seg-
ments [167–169]. The highest adsorption capacities for CBHs are typically obtained for
the lowest crosslink density and vice versa [168]. However, there is a minimum value of
crosslink density, which is necessary to avoid mechanical failure or outright dissolution of
the adsorbent materials [170]. At the point of the lowest crosslinking density, the molecular
weight of the HGs is preferred to be high to ensure that most of the polymeric chains are
bound by a minimum of one covalent bond to the rest of the material. Studies showed
that higher molecular weight CMC HGs have a higher internal volume in the polymeric
chain that causes an increase in the adsorption capacity of the HGs [169]. The surface and
cross-section porous structure of a CBH is shown as a function of crosslinking density
in critical content. The crosslink density of dual crosslinked hydrogel (DCH) was higher
than single crosslinked hydrogel (SCH) because of a decrease in water content during
crosslinking. Due to the increase of crosslink density in the polymeric network, the adjacent

Gels 2021, 7, 30 16 of 28
chains of HG materials came closer, resulting in a significant decrease of the surface and
cross-section porous structure of DCH (Figure 5) [171].
Figure 5. Morphology of single crosslinked hydrogel (HG) (SCH) and dual crosslinked HG (DCH)
specimens: Surface and cross-sectional SEM images of SCH and DCH [171].
4.2. Initial Concentration of Pollutant (ICP)
The amount of pollutants adsorbed on the adsorbent surface is highly dependent
on the initial concentration of pollutant (ICP). In a fixed solution volume and adsorbent
mass, the number of adsorbate molecules is proliferated when ICP in wastewater is in-
creased [172]. Consequently, more adsorbate molecules bind to the active sites of the
adsorbent, thus accelerating the diffusion of dyes or heavy metals onto the adsorbent sites
due to the increase in driving force of concentration gradient, resulting in higher adsorption
capacities [9]. However, a decline of adsorption efficiency due to higher pollutant concen-
tration was reported in some recent works [173,174]. In the typical adsorption process,
adsorption capacity is sharply increased until the plateau state. Afterward, a further in-
crease in ICP does not improve the adsorption process, resulting in a decrease in adsorption
capacity [175]. The following explanation was made in most of the current works: at low
pollutant concentration, the ratio of an initial number of moles of pollutant ions to the
accessible sites of CBH is large, which causes higher adsorption capacity. On the other
hand, at higher pollutant concentrations, the number of available adsorbent sites becomes
fewer, resulting in a decrease in pollutant removal efficiency [160,174]. Wang et al. [155]
studied the influences of the initial concentration of MB dye on adsorption capacity utiliz-
ing MB concentration between 1800 and 2700 mg/L. The adsorption capacity was linearly
increased with initial dye concentration until a plateau was achieved at 2500 mg/L. At a
concentration above 2500 mg/L, the adsorption capacity started to decrease with increasing
concentration. Recent studies on other CBHs for the removal of dyes and heavy metals
also reported a similar trend of adsorption capacity with respect to ICP. The adsorption
capacities of different CBHs as a function of initial concentration are summarized in Table 4.

Gels 2021, 7, 30 17 of 28
Table 4. Effects of initial pollutant concentration on adsorption capacities of CBHs.
Materials Dye/Metal Initial Concentration (mg/L) Adsorption Capacity (mg/g) Ref.
Cellulose-based porous adsorbent Methylene Blue
3000
2500
1500
500
1505.2
1471.5
1175.4
237.7
[175]
Lignocellulose-based nanocomposite
hydrogel
Methylene Blue
2500
2200
1800
1975
1875
1710
[155]
Carboxymethyl-based cellulose Methyl Orange
1500
1000
500
1825
1650
950
[176]
Chitosan/cellulose hydrogels Congo Red 500 165 [55]
Pineapple peel CBH Methylene Blue 200 150 [55,177]
Cellulose-based adsorbent
Cd2+
600
1000
1600
2000
350
460
530
540
[124]
Pb2+
600
1000
1600
2000
420
630
780
810
Ni2+
600
1000
1600
2000
200
320
350
360
Carboxymethyl Cellulose
Zn2+ 200
500
90
170
[148]
Pb2+ 200
500
65
110
4.3. pH at the Point of Zero Charge
The pH at the point of zero charge (pHPZC) is a critical parameter for the adsorption
process that can change the chelating ability of adsorbents by affecting their swelling ability
and interactions between adsorbents and ions [156]. When the pH pHPZC, the adsorbent
surface is positively charged because of an increase of H+ concentration in the aqueous
solution (protonation). Hence, strong electrostatic interactions occur between the positively
charged adsorbent surface and anions. Conversely, the aqueous solution is deprotonated
at pH pHPZC, creating a negatively charged surface that interacts with cations [117–119].
The protonation and deprotonation mainly occur at different functional groups, such as
carboxylic [105] or amine [174,178], and the precipitation of ions in HGs. Typically, the
adsorption capacity of CBHs for heavy metal ions is higher at the basic pH of the solution;
in fact, there is a range of pH values for each metal ion wherein the maximum adsorption
occurs. However, at pH 7.0, metal ions interact with excess OH− in aqueous solution and
precipitate as metal hydroxides form, thus impeding the adsorption process and reduce
the adsorption capacity of HGs. The typical pH used for optimum metal adsorption on
CBH materials is about 5.0–6.0 [160,178,179]. Recently, Amr El-Hag Ali [156] investigated
the adsorptive nature of CMC as a function of solution pH for the removal of heavy metals
such as Co2+, Cu2+, Fe3+, and Mn2+ from wastewater. Results showed that the adsorption
capacity of CMC HGs increases at higher pH values. Excessive H+ at extremely low pH
values compete with metal ions to cohere active sites of the CMC, causing a lower uptake of
metal ions. The same adsorption protocol is applicable to eliminate dyes from wastewater
using CBHs. For instance, the maximum adsorption capacity of a novel CBH for the
removal of Congo Red (CR), an anionic dye, was obtained at pH ≈4.0, and the adsorption
capacity was declined with increasing pH values. At higher pH, the excess OH− covers
active sites of adsorbent molecules that limit the adsorption of CR dye molecules [82].

Gels 2021, 7, 30 18 of 28
Conversely, cationic dyes are adsorbed on the adsorbent surface at higher pH values such
as metal ions [116]. Table 5 includes some examples of pH values required to maximize the
adsorption capacity of various CBHs.
Table 5. Effects of pH on adsorption capacities of different CBHs.
Materials Dye/Metal pH for Max. Adsorption Ref.
Porous cellulose-based bio-adsorbent Methylene Blue (cationic dye) 9.0 [46]
Carboxymethyl cellulose Cd2+, Pb2+ 4.0 [180]
Cellulose–graft–acrylic acid HGs Cd2+, Pb2+, Ni2+ 3.0 [124]
Amide-functionalized cellulose-based
porous adsorbent
Acid Black (anionic dye)
Acid Red (anionic dye)
Cu2+
2.0
2.0
7.0
[181]
4.4. Temperature
The adsorption capacity of CBHs greatly depends on temperature since the kinetic of
adsorbate molecules in the aqueous solution is significantly changed when the tempera-
ture is raised. In the endothermic adsorption process, the adsorption capacity of CBHs is
increased with the increase of temperature, which is the opposite of exothermic adsorp-
tion [100,182]. Typically, the adsorption capacity of CBHs for the removal of dyes from an
aqueous solution is proportionally increased with temperature [46,183,184]. According to
Lin et al. [46], the mobility of dye molecules is notably increased at higher temperatures,
providing a higher potential to enhance the interactions between dye molecules and the
adsorbent surface. As a result, the dye desorption from the adsorbent surface is minimized,
leading to a higher adsorption capacity. Additionally, a swelling effect within the CBH
structure may initiate due to the increase of temperature, causing the further penetration
of dyes onto the adsorbent surface [182]. However, the adsorption capacity becomes in-
dependent of temperature when equilibrium is achieved (Figure 6B; blue bar). Similar to
non-CBH adsorbent (Figure 6A), the adsorption capacity of CBH adsorbent is slightly fallen
with an additional increase of temperature after equilibrium (Figure 6B; red bar) [184].
When adsorption takes place at a temperature higher than equilibrium temperature, the
desorption characteristics of the CBH molecules become dominant, owing to the exces-
sive molecular motion that causes such a slight decrease of the adsorption capacity [46].
Furthermore, similar to adsorptive removal of dyes from wastewater, CBHs show higher
adsorption capacity for the removal of metal ions through endothermic adsorption [185],
while the adsorption capacity is significantly lower for exothermic adsorption [186].
Figure 6. Effect of temperature on adsorption capacity: (A) adsorption kinetics of Reactive Red 189 dye on crosslinked chitosan;
pH = 3.0, initial dye concentration = 3768 g/m3, adsorbent size = 2.3–2.5 nm, and crosslinking ratio = 0.2 [184], (B) percent
adsorption of Acid Blue and MB dyes on CBHs [46].

Gels 2021, 7, 30 19 of 28
4.5. Ionic Strength
The ionic strength of an aqueous solution refers to the concentration of ions present
in that solution. The ions in the solution are usually formed by dissociation of salts when
dissolved in aqueous medium. In other words, the more salts in a solution, the higher the
ionic strength of that solution [187]. Typically, industrial wastewater contains a wide variety
of salts, such as NaCl, KCl, NH4Cl, CaCl2, MgSO4, AlCl3, etc. along with other organic
and inorganic pollutants, and these salts show strong influences on the adsorption capacity
of CBHs during wastewater treatment [188]. The relationship between ionic strength and
adsorption capacity is mostly studied for dye removal from aqueous solutions. Liu et al. [46]
demonstrated the effects of ionic strength on the dye removal efficiency of acrylic acid and
acrylamide grafted CBH, where they used a different amount of NaCl salt to change the
ionic strength of the solution. According to their study, the dye removal efficiency was
decreased with increasing NaCl concentration, due to the competitive effect between the
salt ions (Na+ and Cl−) and the existing dyes with functional groups (–COO−, –NH3
+,
and OH−) on the CBH surface. With the growth of NaCl concentration, the shielding
effect of Na+ and Cl− ions for the ionized dye molecules was improved, which causes
the reduction of adsorption efficiency of adsorbents [189,190]. In addition to NaCl, the
absorbency of CBH was also decreased with a higher concentration of other salts, including
KCl, NH4Cl, CaCl2, and AlCl3. When compared among these salts, the adsorption capacity
of CBH in the existence of monovalent cations reduces in the following order: NH4+
K+ Na+. However, when compared based on ion valance, the highest declination of
the swelling capacity of CBH was observed in the presence of trivalent cations with the
following descending order: trivalent divalent monovalent cations [191].
4.6. Coexistence of Ions
The coexistence of various ions in the aqueous solution has mixed impacts on the
adsorption efficiency of HG for each ion species. When multiple ions present in the solution,
some of the ions either decrease the adsorption of others due to competition or collectively
increase the adsorption through cosorption [192,193]. Antic et al. [193] investigated the
sorption of Pb2+ in the presence of other ions, including Ni2+, Cd2+, Cu2+, Zn2+, and Co2+.
In the binary system, the adsorption of Pb2+ was decreased by 5.27% due to competition,
and it further decreased to 11.1% when the tertiary system was used. Furthermore, the
existence of interference ions such as K+, Na+, Mg2+, and Ca2+ in the solution causes
competition with heavy metal ions, such as Cd2+, Cu2+, and Pb2+ around the same surface
of the adsorbent molecules, impeding the adsorption of heavy metals [194]. Sharing the
same binding sites of HGs by existing ions in the solution is another potential cause for
such low adsorption of heavy metals [192,195].
Moreover, the functional groups (i.e., –COOH, –NH2, etc.) entrapped in HG poly-
meric chains have shown selective adsorption for certain ions based on ion properties,
including ionic radius, electronegativity, and ionization potential. For instance, –COOH
in a nanocomposite HG has higher adsorption selectivity to Pb2+ compared to Ni2+, Cd2+,
Cu2+, and Zn2+ due to the difference in the above ionic properties [196]. The metal ions
show more propensity to HGs compared to other ions when the radius of metal ions is
relatively higher. So, the bigger the ionic radius, the superior the binding capacity [193,195].
In addition to ionic radius, ion hydration radius also plays a vital role in metal ions removal
efficiency and adsorption capacity. The literature showed that the change in adsorption
capacity is very minimal for the ions with smaller hydration radius, even though the
solution consisted of different interfering ions such as Na+, Ca2+, and Ba2+ [197].
In addition to that, the alternation of functional groups in the CBH structure has great
influences on the adsorption selectivity of the adsorbent for specific metal ions. Amr El-Hag
Ali investigated the simultaneous adsorption of Mn2+, Co2+, Cu2+, and Fe3+ on CMC/2-
acrylamido-2-methyl propane sulfonic acid (AMPS) HG derived by γ-radiations-induced
copolymerization and crosslinking. He functionalized the adsorbent structure by altering
the ceoncentratio of the AMPS. The uptake of metal ions for various AMPS concentration

Gels 2021, 7, 30 20 of 28
is summarized in Table 6 that revealed that the adsorption capacity of the CMC/AMPS
adsorbent significantly increased with increasing concentration of the AMPS. One of the
reasons for such behavior is solution pH. Typically, the pH of the solution changes when the
functional groups in CBH structure are altered [198]. The upsurge of AMPS concentration
in the HG directs to the increment in the dissociated groups and subsequently risese the
electrostatic repulsion, resulting in the expansion of the network structure.
Table 6. Effect of carboxymethyl cellulose (CMC)/2-acrylamido-2-methyl propane sulfonic acid
(AMPS) composition on the adsorption of heavy metals.
AMPS Content (wt%) Swelling (%)
Amount of Metal Ion Recovered (mg/g)
Co2+ Cu2+ Fe3+ Mn2+
10 581 16.3 27.4 25.3 7.1
25 617 43.1 52.7 56.8 18.6
50 690 60.6 75.3 80.4 46.8
5. Conclusions and Future Outlook
Dyes and heavy metals released by various industries are among the most common
pollutants of wastewaters, which have a detrimental effect on the environment including
aquatic lives, human health, and the ecosystem. Therefore, the remediation of pollutants
from the wastewater is imperative for a safer environment. The adsorption process in-
volving different types of adsorbents is considered as an effective and efficient wastewater
treatment method. However, most of the adsorbents used for the treatment purpose are
synthetic and non-biodegradable, and management of the adsorbent after the treatment
is another environmental concern that triggers the researchers to find and use a naturally
derived and renewable source of materials as adsorbent. Cellulose-based hydrogels (CBHs)
are ideal candidates meeting the requirement with some added benefits such as high re-
moval efficiency, cost-effectiveness, and easy process. This review covers important aspects
of wastewater treatment using CBHs such as the synthesis of CBHs, adsorption mechanism,
and parameters to optimize adsorption capacities, which have barely been covered in the
literature. In addition, based on the limitations in the literature covered, the following
scopes are recommended to address for future research consideration.
• Most of the literature covered treatments of lab-based wastewater containing a single
pollutant, instead of real industrial wastewater. Some reports claimed that the presence
of multiple ions in the wastewater may influence the adsorption of any specific
ions [192,193]. Some wastewaters contain additives or other auxiliaries (such as salts,
surfactants, etc. in textile wastewater) besides the target pollutant, which might have
some potential impacts on the adsorption efficacy, and hence need to be explored.
Therefore, extensive future work is needed to investigate the adsorption performance
of CBHs in the real industrial wastewater system. The primary attempt could be the
pilot-scale treatment of the industrial wastewater or at least simulated wastewater
containing multiple pollutants that mimic real industrial wastewater.
• The greatest limitation perceived by the authors during the preparation of the pa-
per is the lack of clarity and inadequate information on the adsorption mechanism.
Moreover, the adsorption behavior of CBHs for non-ionic pollutants such as non-ionic
dyes, water-insoluble dyes (such as pigments, disperse dyes, vat dyes, sulfur dyes,
etc.) have been overlooked in the literature. Consequently, more experimental and
theoretical research is a pressing need to comprehend adsorption mechanisms that
might potentially help unlock and identify the most effective mechanism.
• Many cellulose-based hydrogels lose their adsorption capacity after regeneration.
Some reports revealed that the hydrogels retained the adsorption capacity only when
regeneration is conducted with caustic soda. Moreover, modifications (both physical
and chemical) and pretreatments of cellulose can enhance the adsorption capacity

Gels 2021, 7, 30 21 of 28
of the CBHs to some extent. Several chemical and physical networking approaches,
such as modification with graphene oxide (GO), nanoparticles (NPs), carbon nan-
otubes (CNTs), and carbon quantum dots (CQDs), and blending with other suitable
synthetic or natural polymers can be tested to enhance the gel characteristics as well
as regeneration performances.
• The stable structure and effective swelling of CBHs are crucial for wastewater treat-
ment especially at the condition of elevated temperature of the industrial waste
stream. Some CBHs tend to weaken and lose their mechanical strength upon repeated
swelling. A chemically crosslinked network often improves the stability and adsorp-
tion performance of the hydrogels. Moreover, the incorporation of magnetic particles,
nanoparticles, and different chemical catalysts should be investigated to enhance the
adsorption capacity and swelling properties of the CBHs. It is important to explore the
scope of improving the mechanical durability of CBHs with increases in self-healing
ability after a swollen state.
• The overall physiochemical composition and morphology of the CBHs mostly dic-
tate their performance in the area of industrial wastewater treatment. A designed
formulation and optimized synthesis conditions are critical parameters in designing
a specific CBH overcoming the challenges and shortcomings such as low turnover
number, lesser resistivity, and mechanical strength. In addition, issues in thermal
stability, swelling ratio, and pH sensitivity still need to be addressed for its full-scale
implementation.
Supplementary Materials: The following are available online at https://www.mdpi.com/2310-2
861/7/1/30/s1, Table S1: Recent works on removal of heavy metals using CBHs, Table S2: Recent
works on removal of dyes using CBHs.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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